1. Field of the Invention
This invention relates to a shield for high energy radiation particularly one which enables the build-up of an electric field which acts to attenuate the penetration of such radiation.
2. Description of Related Art
The present invention relates to radiation shielding, and, in particular, to shielding using insulating materials.
A shield is placed between the source of radiation and a sensitive element for the purpose of reducing the irradiance upon the sensitive element. The shield attenuates or reflects some of the radiant energy so that less radiation arrives at the sensitive element. Usually one finds that increased shielding material either in depth or density provides increased attenuation. Until recently, the penetration of high energy (HE) electrons through slab shields has been modeled without including the effect of electric field on the trajectory of the electrons. Experimental data on the development of the electric fields in irradiated insulating polymethylmethacrylate (PMMA) slabs provided the impetus for developing a Monte Carlo version of electron transport code that includes the effect of electric field. Several experimenters had observed that penetrating dose is decreased as the electric fields accumulate, but a full parametric study for these observations has been unavailable. The lack of a complete model relegated the experiments to interesting observations that could not be extrapolated to other situations.
Theoretical models of space charge evolution and dose in irradiated insulators at high electric fields are improved by the inclusion of the effects of the electric field on the scattering of fast electron. Consideration of these effects impacts on the development of the dose, charge, fast electron current and total current. Each of these items is calculated in computer simulation with its full time and spatial dependence.
New measurements by others have provided electric field and charging data on insulators irradiated by high energy (HE) electron beams. Previous predictions of electric field and charge distributions as a function of time have not included the effects of the electric field on the motion of the HE electrons. For electrons well below 100 KeV, where the material stopping power is large, the effects of the electric field are less important. It has been observed for electrons above 1 MeV that their trajectories are foreshortened in charged insulators.
Recent experimental data has one striking difference from the previously published theoretical data for partially penetrating electron beams. In the old theoretical data the charge centroid was predicted to move away from the surface through which the monoenergetic beam entered as irradiation progressed. The old theory predicted a much deeper depth for the charge centroid than the recent data showed. Additionally, the old theory predicted only a small sharpening of the spatial distribution of stopped electrons as time progressed. Instead, experiments found that the centroid of charge moved rapidly toward the entrance surface and developed a sharply defined peak of stopped electrons. It was obvious that the effect of the electric field on the HE electrons must be included in any good theory.
Taking into account the electric field build-up in Monte Carlo simulation on the motion of the HE electrons gives better agreement with experiments than do the older models.
The prior theories for estimating both the distribution of space charge and the internal electric fields were obtained using the procedure as disclosed in xe2x80x9cRadiation Induced Electrical Current and Voltage in Dielectric Structures,xe2x80x9d AFCRL-TR-74-0582 (1974) by A. R. Frederickson.
The combination of a Monte Carlo calculation including electric fields to calculate HE current and the prior theory to calculate total current yields improved predictions of electric field build-up in a dielectric slab as a function of time. This is of concern when the incident beam energy exceeds 50 Kev and the total dose exceeds about 50 krads. The improved predictions, in agreement with experiment, predict that the charge centroid moves toward the incident surface, instead of away from it as is the case if the field effects on HE electron motion are neglected. The improved predictions, also in agreement with experiment, indicate that the concentration of charge sharpens to a more narrow peak as time during irradiation progresses.
As a result of the development of the electric field, the amount of radiation dose which penetrates to depths within the insulator and beyond is strongly changed. At most depths the dose is decreased by the electric field and therefore the insulator acts as an improved radiation shield. Thus, there exists a need for an improved shielding device and a process for determining the performance of the improved shielding device.
In this invention, previous high energy radiations into the shield produce high electric fields which, in turn, provide increased attenuation to subsequent high energy photon and electron radiations. The use of the electric field has the advantage of increasing the attenuation of high energy radiation without adding extra weight to the shielding material.
Therefore, one object of the present invention is to provide a process for developing electric field and charge storage in insulators acting as radiation shields with approximately a factor of two improvement in shielding effectiveness compared to non-insulating shields.
Another object of the present invention is to provide a process to more accurately predict electric field levels which would indicate approach of breakdown threshold.
Another object of the present invention is to provide a process for better understanding the breakdown of space borne electronic insulation due to HE electron radiation belts.
Another object of the present invention is to provide a process for calculation of shielding effectiveness for insulating shields for any radiation composed of high energy electrons and/or photons in order to predict for cases which cannot be experimentally studied.
Another object of the present invention is to provide a process to protect external electrical circuits from the effects of spontaneous electrical breakdown pulses that may occur in the insulator shield.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the pertinent art from the following detailed description of a preferred embodiment of the invention and the related drawings.